In a vehicle, electric loads are directly connected to an electric power generator which is coupled to an engine. When a generated voltage is changed in order to raise the regenerative effect of electric energy, for example, when the generated voltage is raised at a time of vehicle deceleration, a change in the brightness of a headlight or the like is caused from the voltage fluctuation, or the life of the headlight is shortened.
As a countermeasure against this, electric power is supplied to an electric load through a DC/DC converter. However, a large-capacity chopper-type DC/DC converter, which can feed electric loads of the whole vehicle, has a large size and is expensive.
As a low cost power supply system, it is proposed to supply a generated voltage to an electric load through a series regulator. Power supply efficiency at a time when a voltage is adjusted by the series regulator is proportional to a voltage difference between the input and output of the series regulator. Here, the power supply efficiency is the ratio of output power (supply power to the electric load) from the series regulator to input power to the series regulator. As the voltage difference between the input and output of the series regulator becomes large, the power supply efficiency is reduced.
That is, the power equivalent to the reduction in the power supply efficiency is consumed in the series regulator and generates heat. Therefore, in the case where large power is supplied to the electric load, when the operation is performed at a point where the voltage difference between the input and output is large and the power supply efficiency is low, the heat generation of the series regulator is large. Thus, an erroneous operation or a failure of component parts of the circuit occurs.